Demodulator for frequency modulated waves



Patented May 25,19 2,441,957

7 23413957 t DIE-MODULATOR FQRFREQUENGY MODULATED WAVES Louis A del {p55, Staten Islan N j;, as siij1 rto Federal Telephone and Radio olffidrafidhi Newark; N: J., a'corporatiofl afiielawsre Application-November 13, 19 12, SeriaLNo. 465,451 I invention relates to radio and more tierindicated by the 'waveiia. The wave 'sfiow'ti tibifldrlytb 'diinodtilltil'jiil-S'S StifiXIS'. 7 t0 Viil'yin' amplitude aswell as'inrreque'ncy to fur- Oheofth'e objeot's of'thisi'n've'rition is'to'proiiide the'r i1hist'raixa. the effets "of amplitude vlriaitidri 'almetifod and means [to effect demodulation of in the 15u1s'es'l2'd; Frequency modiilaltiiirileilso-p o--' energy fro'n'i a. frequency motdulaited'wave source. duce's some variation in the" ampiitiide 6f. the Afiqil'hfiji 11103151111aifd wave may be Caused 15111865'l2a, biiifsind'th'riidiliilfitidfisifi fiiquii'by purposely orotherwi'seto haifeyari'ations in am? arrusuaily of very's'man per'eehta'gfe, the' ef feet phfiiid aswell as freqfincy; These ya'ria'tions'in thereof in 'a'mplitudebf the'puiss l f lfis'ffieg l ihljel amplitude, however, render it diflieult to obtain The amplitude variations of pii1ses"|2d,r g'ar'dl'ess 'accuratefaemoduiation bf "the fre uency modiiia- 11) D'fbdiif'sli, are"satisfactorily'removeafw myifiethtion signals; fic'oordi'ngiy, it is another object od'jof demodulation. I I army invention to, firovidei'for" the demodulation The pulses 2a are next 's'u'bjectedto'"a trarislttt offr'equency modmactedwavs' whereby effects ii'i'g processincludingthebperationof'thtii diiits due to'amplitud variations'presem'mafrequency T4; Hi and [8; The piiis'esl'ztz'ar first pissed modulatedwave are'greaitly fifinimiz'edii riot eh- '5 through a, trigger'circuit l4 soas to produce a sec tire'l eliminated"during-"tire demn'amatio rocon'd ta i'gmar wav- Ha Havin sfibstthtidlly '65s. Vertical sides'erid equair ampfimde; AS'he1ih- Thea'bove objeets' and other's ancillj'ary thereto, 7 'afterdescribedifidetaiLthecireiiit"f iiserraihgd wilFbecome more apparent fromtheironowin 't 'd suc e sively-from fi s t f' pdetai'leddescriptiontoibe read'in connection with 20 er'ait'ionitu'anotheffby one bfiihe -pi 211 i ind Fig.2"i-s a; graphical iliustrationof-the method tiveiand'niegativ pul fia'qrsuustafltinycom tg -ofthein ntion; 2Y5 'st'ant amplitude areproduced, fl heerripiiftude Fig. Bi'sascheindticdiagram of a' compoi ent vari s ppea'rmgun" i ifl d mf hje part'ofth'e system showri' iiiiFig. 1; I pulses" l 2a are thdseiiniihaitedf Bypaissingfthe Fig. "4is "ai-blok diagram otandther forth of pulses fi ou h a reetifiefl'da yetrairiof demodulation system; and" positive constant amplitude "pulses 8:; is rpm;

Referring "t'o'Figs. l" and '2, the system shown h 'f s P1115655, vare, a 'efi i o h allow" thereintis particularly"adantedto demodulat'e a ne n n f ated t ereby stea ing irequency'modulatea wave having presentltheree I nf u q 'WQJVQ'H Wh h fl wim 'thqf fi q fi y in amplitude modulationeor at least variations ,3 'ifi e hbm q n fm iflfit d WW?? ofi'ampiitudewherby the distortion effects of hu i e ncy o l fi ignaii e ec d such amiilitude variations are grealtiy minimized i h j i [distortion due to empntude "varia- 'not"entirely 'eii'midatedfrom'the 'finai're'oep- 171025 33 3 h input Wave? v tio'ni output of the. fiequencyrmodulation' sig'rills; mg, 3; I have shqw i a h mai di 0f The'systemtomprises a'kn'oWn' form of iimiterfl u 1 a firlgger-typeqf' m q i i 1 to which energy from a. frequency modulated infor 71158 in m m en i I 6 fi' COII'IpTiSeS put source 8 is. applied,whereintffepeaks 'o'f'the ew i 24 Y T Q Y y;P1 153 5 incoming"frequency'modulated'wavefiareClipped ani pnre wr id 30, 2;} cre ir'gr usisa', oft-st *levels'i I and 1'32 "ispmduces-"a-wave Ina 3 Control grids "cathodes 7 8 ampimia variationsstraws-variation 1d steep 48. One input terminal 63 is connected to sup pressor grids 30 and 32, and theother input terminal 65 is connected to the common conducting thepos'itive "end of voltage source 92 while the means 46. One output terminal 61 is taken from -other'output terminal I81 is connected to plate said connecting means 46 while the other output terminal 69 is taken from plate 28 of tube 24. q

The values of plate resistances 56-, 52,-.bias resistors 64, 66 and coupling resistors 54,58 are not critical. However said coupling resistors and said- 2,441,957 "1 Til :"11':

connected to the negative end of voltage source 92. One output terminal I85 is connected to tubes I61and 'l'8. The grid of one tube may have zero bias or a Unequal bias is' used on the grids of the two small positive bias, the flow of high 1 grid current being prevented by the grid coupling bias resistors may be appreciably larger than said 7 plate resistors, and usually may be equal to one another. The size of coupling, capacitances 56and 68 may be such that the time takenby' them to charge or discharge from an equilibrium value of voltage to the other value is large in comparison with the duration of the triggering pulse but small in comparison with the time betweensuccessive pulses. Condensers of 50 m. m. f. capacitance are satisfactory. V V j v V The circuitis sensitiveto triggering voltage of either polarity. Of course, reliable triggering necessitates the use of triggering pulses of very short duration. The operation of this circuit will be readily understood by those skilled in the art.

Vention which is particularly useful where-amplitude variations in thefrequ'ency'modulated wave are either not presentjor are ofnegligible value. Such a frequency modulated Wave source is applied to a limiter 68' and then passed through a differentiating circuit'lllin' the same manner as explained above. Now, however, differentiated pulses obtained from circuit ID are applieddirectly to a flip-flop circuit 12 which in eifect produces a translation of positive and negative pulses to. unidirectional pulses. The circuit 72 maybe merely an unbalanced trigger circuit wherein pulses of constant'duration are initiated by} single triggering pulses. Such a circuit may be triggered from one equilibrium state to an- In Fig. 4, I show a simplified form of my 111- other, but returns to the originarequilibrium state aftera short time interval thereby producing a series of pulses 13.,Fig, '2, of adesired area each. The pulse output of the circuit I2 is appliedito a low-pass filter I4 for integration, resulting in an audible output wave 2I. The wide duration of the pulses I3 in contrast to the narrow pulses I8a are preferred since they'present more power .per pulse and thereby produce a smoother audible output. 1

In Fig. 5, I. show a suitable flip-flop? circuit which may be'employed as the circuit 12. Such a circuit'comprises two tubes 'I6and'l8 having respectively plates 88, 82,. control grids 84; 86, and cathodes 88,86, the latter being connected together. The plate 86 of tube I6 receives its potential from a voltage source 92 through plate resistor'84. Thejplate 82 receives its potential from the same source 92 through a plate resistor 86. The plate 88 of tube I6 is connected through ajcofupling resistor 9811c the grid 86 of tube'l8. The plate 82 'ofthetube' 78 is connected through a 'coupling condenser I00 'tc the grid 84 of tube 16.115 .volta e'..cliyider J02. is j conne ted across resistor. q The equilibrium of the circuit returns to its'normal state after triggering since one of the plate coupling resistors normally provided in a circuit such as shown inJFig. 3 is herei'n eliminated, and .a comparatively large'value of coupling capacitance is used. The time interval of 'su'cha flip-flop. circuit is deterrnined'principally by the magnitude of the resistor, I88 and capacitance I00, and to a lessenextent, by the value of resistors 96, 98, andjbythe supply voltage 92. I

The operation of the system shown in Fig. 4 is very similar to that shown in Fig. 1 exceptthat the trigger circuit It, the difierenti'ating circuit I 6 and the clipping circuit I8 of Fig. 1 is replaced in Fig. 4 by the multi-vibrato'r, circuit. 72. The output pulses I3 of the circuit '12 are approxi-. mately rectangular in shape, and substantially constant in duration and 'area. These pulses occur at the beginning of each positive half cycle of the input signal, It will be necessary, of

course, that thetime constants of the circuit be so adjusted that the pulse duration be less than the duration of one half cycle of the highest line frequency tobe received. j v

It will beclear from the foregoing, description that the flip-flop circuit 72 may be substituted It will also be understood that ;the individual components illustrated .and described herein are merely given by. way of illustration and are not to be regardcd as limiting.the Scope of the appended l s- 1 1j fl., i g 1 WhatIclaim is: v v 1 .,;'A methodofdemodulating a frequency rnode ulatedwavecomprising clipping the wave toproduce -'a wave of approximately rectangular form, differentiating the rectangular wave" to produce i e-{ 'p0 itive;;andn gative pulses ;one :each for each cycle ofthe wave, said pulses beingdis placed inaccordance with the frequency Hmodu: lation ofsaid-wave,- translating'saidpulsest into unidirectional pulses o f; substantiallyicpnstant .amP t ea d' bst n ia y Q DS BtJWiGt th Q RQP W; Wi Va yi their epet ion r e e ilin l -the re i en r me t i .o said wave...aiidl t'g rating withrespcqtidtime the unidirectional pulses to produce an audible output wave.

2. A system comprising wave source means to clip source to produce a second wave of approximately rectangularly pulsed form, means to differentiate the second wave to produce alternate positive and negative narrow width pulses one each for each half cycle of the second wave, said pulses being displaced in accordance with the frequency modulation of said wave source, means to translate said pulses into unidirectional pulses of substantially constant amplitude and substantially constant width, the unidirectional pulses varying in their repetition rate according to the frequency modulation of said wave, and means to integrate with respect to time said unidirectional pulses to produce an audible output Wave. 7

3. The system defined in claim 2 wherein the translating means include a circuit having one mode of stability so that when it is triggered from one state of operation to another it will return to the first-mentioned state of operation after a predetermined interval, whereby pulses of a predetermined constant width can be produced which when integrated with respect to time produce a smooth audible signal.

4. The system defined in claim 2 wherein the translating means include a triggering circuit operable in response to the alternate positive and negative pulses to be triggered by one of the pulses from one state of operation to another and to be triggered back again by the next succeeding pulse thereby producing a second rectangular wave the side slopes of vertical, means to differentiate said second rectangular wave to produce alternate positive and negative pulses, and means to rectify the lastnamed pulses. v

5. The method defined in claim 1 wherein the translating f the alternate positive and negative a frequency modulated the waves of said wave which are substantially pulses into unidirectional pulses comprises producing a second rectangular wave having substantially vertical side slopes, a fixed and constant wave amplitude, and a recurrence frequency equal to that of the frequency modulated wave, differentiating said second rectangular wave to produce alternate positive and negative pulses, and rectifying these pulses to produce said unidirectional pulses.

6. The method as defined in claim 1 wherein the translating of the alternate positive and negative pulses into unidirectional pulses comprises producing a second rectangular wave, the sides of each half cycle thereof being substantially vertical, differentiating said second rectangular wave to produce alternate positive and negative pulses of constant amplitude, and producing therefrom constant width pulses the duration of which is controlled.

LOUIS A. on ROSA.

REFERENCES CITED OTHER REFERENCES Ultra-High Frequency Techniques, by Brainerd et al., 1942, pages 17617'7. (Copy in Div. 51.)

Journal of the Institution of Electrical Engineers, June 1942, page 110. 

